Despite recent advances in surgery, radiation, and chemotherapy, malignant gliomas remain universally fatal, with a median survival of 12-15 months for glioblastoma and 2-5 years for anaplastic astrocytoma. Limitations in neuroimaging complicate the clinical management of patients with gliomas, and impede efficient testing of new therapeutics. Amide proton transfer (APT) imaging is a novel molecular MRI technique that generates image contrast based on the endogenous cellular proteins in tissue. This APT imaging project has been extremely successful during the first grant period (7/09-4/13). We developed several relatively fast three- dimensional APT imaging sequences at 3T and 7T and established several effective image acquisition protocols and image processing approaches for imaging of human brain tumors. In addition, we demonstrated that APT imaging is a highly valuable addition to the MRI armamentarium for the more specific characterization of human brain tumors. Notably, our preclinical study in various glioma models and models of radiation- induced necrosis in rats clearly showed that active glioma (hyperintense) and radiation necrosis (hypointense or isointense) exhibited opposite APT-MRI signals, and could thus readily be distinguished. These preclinical results are exciting, and, if validated with appropriately powered studies in patients, the implications for clinical care and experimental therapeutics will be enormous. The ultimate goal for APT imaging is its standard use in a clinical setting on a variety of MRI systems from different vendors. However, clinical APT-MRI experiments are often limited by scanner hardware constraints (particularly amplifier duty cycle and pulse length) and specific absorption rate (SAR) requirements. Therefore, current APT imaging protocols vary substantially among different institutes, far from being optimized, and the results acquired from different research centers are difficult to compare to one another. The overall goals of this renewal application are to refine this important protein-based molecular MRI technology into a sensitive, user-friendly, and clinically reproducible approach and to demonstrate its potential to help resolve several diagnostic dilemmas in brain cancer therapy. Based on the results obtained in the previous grant period and recent progress in the field, we have designed the following three specific aims: (1) implement and optimize the parallel RF transmission (pTX)-based APT-MRI technique at 3T; (2) evaluate the clinical value of APT-MRI in identifying non-Gd-enhancing high-grade gliomas; and (3) quantify the accuracy of APT-MRI in distinguishing pseudoprogression from true tumor progression in GBM treated with chemoradiation therapy. If this step of the APT investigation is successful, this research will provide the optimized approaches and critical sensitivity and specificity data required to translate this important APT-MRI technology into the clinic.